1. Field of the Invention
The invention relates to the field of radiofrequency identification (RFID) tags and in particular to protecting RFID tags against high field burn out or overload.
2. Description of the Prior Art
In the normal operation of an RFID label, a voltage is generated in a field by applying an AC signal to a tuned transmitting antenna in a reader (or coil for low frequency operation). An RFID tag receives the power through a tuned antenna (or coil) when placed in the proximity of the generated field. When the tag (receiver) is placed in close proximity to a reader (transmitter), the tag can receive so much power that the voltage on the RFID chip must be limited so as not to damage the RFID chip. Normally, this is accomplished by placing a DC regulator on the RFID chip. The regulator must be of sufficiently low impedance so as to control the voltage even when the power supplied from the reader might generate many volts.
For high frequency operation utilizing E field transmission, the voltages supplied to a chip might reach 15 or 20 volts while the chip is trying to regulate below 5 volts. Unfortunately, for low frequencies where coils are used to transmit energy utilizing H fields, the voltage on the front end of the chip can easily reach voltages in excess of 50 volts. Regulation by limiting the voltage to only a few volts can cause severe problems. For a typical coil impedance of 200 ohms and regulation at 5 volts means that the power sustained on the chip becomes greater than 1 watt. Without any heat sinking capabilities, a small RFID chip will literally burn up.
There are a few techniques which one can implement to minimize the problem. First, the chip can allow for higher voltage operation without drawing excessive currents. Unfortunately, more state of the art integrated circuit technologies allow only very low voltage operation. Placing a low current limited in series with the main circuitry of the chip is a possibility, but once again an active device such as a transistor cannot be built into the chip and sustain the high voltages. The other option is to place a passive device such as a resistor in series with the chip. However, operating the chip at low voltages (long read distances) becomes quite difficult if not impossible.
What is needed is an apparatus and method which overcomes the inadequacies of the prior art solutions.